Overview doc of cubic spline theory

[pmk-LANL]

Create a page that goes through the main points described in the Barnak et al paper on cubic spline unfolds.

• How cubic splines are defined by their knot points, and what moving these knot points does.
• Assumptions/boundary conditions.
• How cubic splines remove energy dependence and therefore results in an invertible matrix to solve for x-ray spectrum.
• How uncertainties are propagated, including covariance terms due to the various channels having overlapping responses in photon energy space.